1. Field of the Invention
The present invention relates to a device for purging a high-output polymer extruder.
Starting up a polymer extruder entails having previously purged the extruder. The purpose of this purging operation is not only to avoid contamination of the extruded product but also to optimize the conditioning of the polymer before production begins.
2. Discussion of the Background
It is known practice for such start-up purging to be performed using a purge valve with which the extruder is equipped upstream of the die. At the time of purging, the valve does not deliver into the die but diverts the molten polymer on to a slide which guides the sausage of still rather inconsistent and very sticky polymer on to the ground, which has been wetted in preparation.
On a high-output extruder, a purging operation such as this entails the participation of numerous operators. The first operators use shovels to cut the sausage of polymer into blocks of about 15 kg and quickly push these blocks away from the slide discharge zone. Other operators use their shovels to transfer the blocks to a cooling zone, where they can cool and solidify in the air. The cooled blocks are then loaded manually into a skip.
To guarantee optimum extruder start-up and make it possible for production to be regulated effectively, it is important to have a purge rate close to the nominal throughput rate. Now, on modern extruders used for granulating polymers, the polymer throughput on purging may exceed 15 tonnes/hour. It then follows that the operators performing the purging have to work very quickly to remove several tonnes of hot, sticky polymer which are poured out on to the ground by the purge valve in a space of 10 to 15 minutes. This work is not only very demanding in terms of labour and a physically arduous task, but is also dangerous. This is because the operators are constantly exposed to a risk of burns on contact with the high-temperature polymer.
The object of the present invention is to make the operation of purging a high-output extruder less arduous and less dangerous.
According to the invention, this objective is achieved by a device for purging a polymer extruder.
According to the invention, the extruder comprises, in the way known per se, a purge valve allowing the stream of molten polymer to be discharged. According to the invention, a cutting device is arranged at the exit of the purge valve to cut the continuous stream of molten polymer leaving the purge valve into blocks. A conveying device is arranged at the exit of the cutting device for transporting these blocks immediately a certain distance away from the extruder. As a preference, the device is a hydraulic device which transports the blocks in a flow of liquid, generally an aqueous liquid, a certain distance away from the extruder. It will be appreciated that such a flow of liquid is a particularly reliable and effective means of quickly removing the still rather inconsistent and very sticky blocks of polymer. Specifically, the blocks of polymer are straight away cooled vigorously by direct contact with the conveying liquid, which causes almost instant solidification of their outer surface. The flow of conveying liquid effectively avoids the blocks sticking together again or sticking to the conveying device. It then follows that the purge device operates very reliably and requires practically no human intervention in removing the molten polymer in the form of blocks. This device therefore plays a part in appreciably improving the working conditions and safety when purging an extruder. By using the proposed device, significantly fewer operators are needed for start-up purging, it being possible at the same time, to operate with far higher purging throughputs. It should also be noted that the proposed device thus allows better conditioning of the blocks of polymer so that they can be recycled. In particular, it makes it possible to produce polymer blocks of a smaller size and more uniform weight than does manual purging.
The hydraulic conveying device advantageously comprises a block-discharge chute located beneath the cutting device. This chute is therefore equipped with a liquid-injection device so as to create in this chute a flow of liquid for transporting the blocks. Thus, the blocks can be quickly and reliably removed from the purge valve discharge zone.
The hydraulic conveying device may thus comprise a channel for receiving the said blocks cut by the cutting device. This channel is therefore equipped with a device for circulating a liquid so that the blocks are transported in the channel by a flow of liquid and cannot be in contact or block the channel. This channel constitutes a simple, effective and reliable means of conveying the blocks of polymer, which are not yet entirely solidified, to a location distant from the actual extrusion zone proper.
The hydraulic conveying device preferably opens into a device for collecting the blocks. This device is advantageously equipped in such a way as to hold the blocks while allowing the liquid which conveyed the said blocks to pass through.
Such a device for collecting the blocks may, for example, comprise a collecting skip arranged in a pit so that the blocks transported by the flow of liquid drop into the skip. The bottom and/or side walls of the skip therefore advantageously comprise openings for discharging the liquid, through which openings the liquid can flow when the skip is raised. As a preference, the bottom and the side walls of the skip comprise discharge openings.
The cutting device preferably comprises a two-edged knife which is guided in sliding in a support structure fixed on the exit section of the start-up valve. This knife is advantageously actuated by a linear motor, such as a double-acting pneumatic cylinder, for example, controlled by a timing relay. By adjusting the time that elapses between two successive actuations of the cylinder, the size of the blocks of polymer can adjusted to fit the purge rate.